1. Field of the Invention
The present invention relates to a droplet discharge head and an image forming apparatus.
2. Description of the Related Art
An inkjet recording apparatus including a liquid droplet discharge device has been known as an image forming apparatus. Here, in the liquid droplet discharge device, a droplet discharge head is used as a recording head. Examples of the image forming apparatus include a printer, a facsimile machine, a copier, and a combined machine thereof.
The inkjet recording apparatus performs recording by discharging a recording liquid (ink) onto a paper sheet. Here, the sheet is not limited to the paper sheet. The sheet includes, for example, a transparency (an OHP sheet). The sheet means something to which a recording liquid, such as ink, or other liquids can be adhered. The sheet also referred to as a “medium to be recorded,” a “recording medium,” or a “recording paper.” The inkjet recording apparatus can record a high definition color image at high speed. Now, the inkjet recording apparatuses are used for industrial systems.
Such a droplet discharge head includes, in general, a plurality of nozzle sequences. The droplet discharge head also includes a plurality of individual liquid chambers. The individual liquid chambers are in communication with the corresponding nozzle sequences. In general, the individual liquid chambers are in communication with a common recording liquid reservoir (which is referred to as a “common liquid chamber,” hereinafter) having relatively large capacity.
Any image can be formed on demand by discharging the recording liquid onto the recording medium by applying energy to the selected individual liquid chambers. Examples of an element for applying the energy include a piezoelectric element and a heater chip.
Recently, demands for high quality images and high speed printing are increasing. To address the demand for high quality images, in the droplet discharge head, a number of nozzles and density of the nozzles are increasing. Accordingly, spacing among the individual liquid chambers is getting smaller. A frequency of applying the energy is increasing. Additionally, to address the demand for high speed printing, the recording head has been elongated. Recently, a so-called “line printer” is available. The line printer can cover the entire portion in the width direction of the recording medium.
As described above, when the energy is applied to the individual liquid chamber, pressure change is caused in the recording liquid in the individual liquid chamber. The pressure change is transmitted to the common liquid chamber, which is connected to the plurality of liquid chambers so as to supply the recording liquid to the liquid chambers. Accordingly, such pressure changes affect not only the recording liquid in the target individual liquid chamber, but also the recording liquid in the neighboring individual liquid chamber. This phenomenon is referred to as “mutual interference.” The mutual interference may cause unintentional discharging of liquid droplets or an unstable discharging state. Consequently, the mutual interference may interfere with forming a high quality image.
To suppress this pressure change, a configuration has been considered which includes a damper in the common liquid chamber.
For example, Patent Document 1 (Japanese Patent No. 3680394) discloses a configuration in which an elastic portion is formed on one of wall surfaces of a common liquid chamber, in order to improve a damping effect. In this example, an ink manifold faces a damper chamber through a flexible member. The damper chamber is in communication with external air through an air vent.
Unfortunately, the configuration disclosed in Patent Document 1 is disadvantageous for supplying ink to the common chamber. The ink manifold has a substantially cubic shape. At an end portion of the flow, the ink tends to stagnate. Consequently, at this portion, air bubbles, which are mixed with the ink in an ink flow channel, tend to stagnate. The air bubbles flow into a cavity by a certain cause. The air bubbles in the cavity may absorb the generated pressure. Thus, it is possible that the air bubbles prevent liquid droplets from being normally discharged.
Patent Document 2 (Japanese Patent No. 4453965) discloses a structure such that a common liquid chamber is narrowed at an end portion in a direction in which the liquid droplets are discharged, and a damper is disposed on a surface facing the narrowed surface. Here, the damper is formed of a thin film. In this configuration, flow speed of ink increases at the end portion of the common liquid chamber. Accordingly, it becomes easier to supply the ink.
In this example, the damper is formed of an extremely thin metal film or resin film. When this film is disposed at a side of a nozzle surface, it is possible that the thin film is damaged from outside by an external force which is caused, for example, by paper jamming. In order to avoid this, another member, such as a nozzle cover, may be required. Thus, it is possible that the cost is increased.
Further, Patent Document 3 (Japanese Unexamined Patent Publication No. 2004-351811) discloses a configuration, with which an attempt is made to obtain a damping effect by including an elastic body, such as a rubber or a flexible material containing air bubbles, in a portion of a wall surface of a common liquid chamber. Further, in this example, since the common liquid chamber is narrowed in a depth direction at an end portion, ability to discharge air bubbles can be improved.
However, this configuration is achieved by pressing the elastic body (e.g., a rubber) into a depression which is formed in advance in a relatively hard resin (e.g., a thermosetting epoxy resin). In order to achieve such a configuration, an additional pressing process is required. Furthermore, it is possible that the elastic body is removed or missed. Accordingly, such a configuration may adversely affect the discharging reliability. Further, in the configuration, the resin is not so flexible. In order to achieve a necessary and sufficient damping characteristic, for the elastic body, a material is required which has a high damping coefficient and resistance against a liquid to be utilized. Consequently, the choices for the material are limited. This leads to an increase in the cost.
There is a need for a low cost droplet discharge head which can be easily assembled, while improving ease of charging a common liquid chamber with a liquid and an air-bubble discharging property of the common liquid chamber.